Mildew Continued from Page 10
Spinach plants, left, which re- ceive sprinkler irrigation, are infected with downy mildew. Below, internal downy mildew discoloration is shown in a let- tuce core. Growers in the Salinas and Imperial valleys are seeking ways to reduce the spread.
The cyclone spore trap has im- proved monitoring compared to the older spinning rod trap, according to Steve Klosterman, a research molec- ular biologist for the U.S. Department of Agriculture. In the Coachella and Imperial valleys, however, researchers only detect these spores during a short window in the fall and winter. “In the desert, we didn’t detect signifi- cant spores before we had mature crops,” Putman said. Because downy mildew cannot com- plete its lifecycle without a host plant, that raises the question of where the first spores of the year come from in the desert. Previous research revealed that the afternoon winds carry a wall of spinach downy mildew spores north to south the length of the Salinas Valley. But analysis shows that Central Coast spores are not the origin of the disease that appears in the desert every fall. Downy mildew pathogens are crop specific. The spinach disease does not go to lettuce and vice versa, and neither goes to many other host plants. One answer to the riddle could be in- fested seeds: 52 of 299 spinach seed lots studied in recent years by UC researchers tested positive for downy mildew.
The search for seed treatments that are effective and economical continues. Another source of infection could be a dormant form of the disease that lives in the soil without reproducing. “We suspect a thick-walled oospore survives in the soil between seasons,” Putman said. These oospores could spring back to life on volunteer spinach plants, on crop residue or on seedlings when the field is seeded and irrigated for the next crop.
Fortunately, there are fungicides still effective when applied early enough to prevent new infections from de- veloping on lettuce or spinach leaves. Preliminary tests on biological materi- als have been promising. “AgroPro reduced disease incidence in three microplot trials and in the mist tent, and it is registered for use and is in use in California,” Klosterman said. Timing is essential with these materi- als, and weather models can help predict when downy mildew is likely to break out.
With lettuce, three hours of leaf wet- ness when the temperature is between 59 and 70 degrees or four hours when the temperature is between 40 and 59 degrees create the conditions for downy mildew. The weather models are intended to save costs by reducing or eliminating fun- gicide applications when the disease is unlikely to develop. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
CIMIS REPORT | www.cimis.water.ca.gov
CALIFORNIA IRRIGATION MANAGEMENT INFORMATION SYSTEM
For the week May 26 - June 1, 2022 ETO (INCHES/WEEK)
YEAR
3.0
THIS YEAR
2.5
LAST YEAR AVERAGE YEAR
2.0
1.5
1.0
0.5
0.0
MACDOEL II (236)
BIGGS (244)
DAVIS (06)
MANTECA (70)
FRESNO (80)
SALINAS-SOUTH (214)
FIVE POINTS (2)
SHAFTER (5)
TEMECULA (62)
IMPERIAL (87)
THIS YEAR LAST YEAR AVG. YEAR % FROM AVG.
1.36 1.70 1.51 -12
2.08 1.73 1.56 34
2.14 2.05 1.96 12
1.99 2.01 1.79 15
1.81 1.90 1.65 13
1.76 1.79 1.68 4
2.07 2.01 1.81 17
1.51 1.36 1.29 24
1.29 1.25 1.26 8
1.97 2.21 2.15 5
W eekly reference evapotranspiration (ETo) is the rate of water use (evapotranspiration—the sum of soil evaporation and crop transpiration) for healthy pasture grass. Multiplying ETo by the appropriate “crop coefficient” gives estimates of the ET for other crops. For example, assume ETo on June 15 is 0.267 inches and the crop coefficient for corn on that day is 1.1. Multiplying ETo by the coefficient (0.26 inches x 1.1) results in a corn ET of 0.29 inches. This
information is useful in determining the amount and timing of irriga- tion water. Contact Richard Snyder, UC Davis, for information on coefficients, 530-752-4628. The 10 graphs provide weekly ETo rates for selected areas for average year, last year and this year. The ETo information is provided by the California Irrigation Management Information System (CIMIS) of the California Department of Water Resources.
For information contact the DWR district office or DWR state headquarters:
SACRAMENTO HEADQUARTERS: 916-651-9679 • 916-651-7218
NORTHERN REGION: Red Bluff 530-529-7301
NORTH CENTRAL REGION: West Sacramento 916-376-9630
SOUTH CENTRAL REGION:
SOUTHERN REGION:
Fresno 559-230-3334
Glendale 818-500-1645 x247 or x243
18 Ag Alert June 8, 2022
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